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Basic Data Structures

Terminology clarification

Terms have obtained different meanings, depending on context.

Term SQL Database implementation
Key Usually understood to mean primary key. Key is unique. The key of a hash table or tree. Key may or may not be unique.
Primary key Set of columns that uniquely identifies rows. Also, the target of foreign keys.
Index Data structure for fast lookup by key
Primary index Not an option in Postgres.
Secondary index Neither clustered nor sparse. All indexes in Postgres are secondary.

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